The present invention relates to the field of measuring structural properties. More precisely, the present invention relates to characterizing a structure by means of a soundwave that is generated and detected by a light pulse.
U.S. Pat. No. 5,748,318 discloses a system for characterizing thin films and interfaces between such films by measuring their mechanical and thermal properties. In the system described, light is absorbed in the thin layer or in a structure made up of a plurality of thin layers, and the modifications to light reflection and transmission are analyzed. The change in reflection or transmission is used to provide information about ultrasound waves produced in the structure. As a result, it is possible to determine the thicknesses of layers and also several optical properties of the structure.
U.S. Pat. No. 5,748,318 is thus an example implementation of a pump-probe system known to the person skilled in the art. In such a system, the light source is a laser emitting short pulses (e.g. femtosecond pulses) at a fixed wavelength so as to produce a first beam that is split in a splitter into a “pump” beam and into a “probe” beam. Thereafter, the light path of the probe beam or the light path of the pump beam is varied by a mirror that is servo-controlled in position. It is then known that the properties of the structure under the effect of the emitted beams give rise to modification in the reflection (or transmission) properties of the probe wave. In particular, and in known manner, by observing the modification in reflection as a function of time, it is possible to determine echoes that are characteristic of interfaces of a structure. Analyzing the echo signal then makes it possible, by way of example, to deduce the thickness of the material, if the speed of propagation of the soundwave in the medium is known.
In order to increase the number of characteristics that are extracted, and in particular both speed and thickness, the publication Evidence of laser-wavelength effect in picosecond ultrasonics: possible connections with interband transition (Physics Review Letters, Mar. 12, 2001, Volume 86, Number 12) describes the use of a pump-probe device as described above, but associated with a wavelength-tunable laser, thus enabling the wavelength of the emitted signals to be varied.
As a result of these wavelength effects, it is possible to access both thickness characteristics and speed characteristics in certain types of structure. Specifically, as described in the publication A novel approach using picosecond ultrasonics at variable laser-wavelength for the characterization of aluminum nitride films used for microsystem applications (A. Devos, G. Caruyer, C. Zinck, and P. Ancey, World Congress on Ultrasonics (Paris Sep. 7-10, 2003), pp. 793-796 ISBN 2-9521105-0-6), for a structure that is transparent to the probe beam, an acousto-optical interaction appears within the material giving rise to the appearance of oscillations instead of simple pulses observed by echo. Those oscillations are known as “Brillouin” oscillations and they have a period that depends on the wavelength of the probe and on the speed of sound in the material.
Another example of a device making use of Brillouin oscillations is also described in patent application FR 2 887 334.
Nevertheless, the number of characteristics that are measurable using those methods remains limited, and it is not possible in particular to characterize accurately a surface or an interface.